Coffee grinders have long been used in the small restaurant industry to provide specified ground coffee amounts to a brewing apparatus. A typical prior art coffee grinder is provided, for example, with a single hopper which feeds directly into the inlet of the grinding mechanism. Each hopper employed in such applications generally holds a significant amount of beans, e.g., in the range of three to five pounds of beans, only a portion of which is ground in any particular grinding operation. The "portioning" of the beans is a critical operation to the ultimate successful brewing of the coffee. It is important that the portioning of the beans from one operation to another of the grinder remain close to a predetermined constant in order to preserve the consistency of the coffee ultimately brewed from the grounds. The ability of the portioning operation to provide a predetermined weight of coffee beans (a "throw") within accepted error limits consistently is frequently called "repeatability", a ratio of the standard deviation to the mean throw. To ensure good repeatability, various techniques have been employed, which generally involve either timer or volumetric controls or a combination of the two. Perhaps the most preferred is the use of a timer to control the grinding operation so that the grinder operates for a predetermined time period during each operation. Timer accuracy provides a repeatability of no greater than about 1%. The grinder itself will provide a repeatability of no greater than about 5%. It can be shown that the repeatability of the ground coffee portion made under these circumstances should vary no more than about 3 to 5%. Repeatability of coffee portion in this percent range has found general acceptance among users of such coffee grinders today.
With the advent of the dual hopper single grinder apparatus, it has now become necessary to control the repeatability of the coffee which egresses from a selected one of the two hoppers from operation to operation. In one prior art system, the coffee is volumetrically portioned by a hollow, rotating cylinder first filling with coffee beans from the selected hopper, then unloading the beans into the grinder inlet. A timer is set to ensure that the grinder grinds all of the coffee beans unloaded into it. The grinder generally is on a short time period longer that necessary to always ensure the complete grinding of the volume of beans received from the rotating cylinder. It has been found that the repeatability is generally in the range of 2 to 3%. The apparatus depends solely on the accuracy of bean delivery to the grinder as opposed to the timer.
Still another type of dual hopper single grinder mechanisms uses dual carriers each associated with a hopper for carrying the beans from the hopper to the single grinding unit. An example of this may be found in U.S. Pat. No. 4,955,510 to Newnan and assigned to the same assignee as the present invention. As described in the Newnan patent, each of the carriers also acts as a portioning device which are preset to accept certain predetermined volume of beans. This type of grinder mechanism does not depend upon the timer mechanism for repeatability accuracy as long as the grinder operation time cycle is sufficiently long, i.e., as great or greater than the longest time needed to grind the entire volume of beans delivered to it from the carrier. However, in order to change the amount of beans ground it would be necessary to change the available volume of the carrier and then one must ensure the time set of the grinder is sufficiently long to grind the delivered beans.
To provide greater flexibility, shutter mechanisms may be employed as valves to control the flow of the beans. Some of the earlier shutter valve arrangements used two shutters which were spaced apart to define a predetermined volume. The upper shutter would allow beans to flow into the space and then close. The second shutter would then open and the beans would be delivered to the grinder. Again the volume was preset by the choice of the distance between the shutters. Such mechanisms did provide high repeatability.
A refinement on the shutter mechanism is a dual shutter mechanism which combines the volumetric and timer controlled types together as disclosed in U.S. Pat. No. 4,789,106 to Weber assigned to the same assignee as the present invention and U.S. Pat. No. 4,971,259 to Nidiffer. The combination of both volumetric and timer controls allows greater choice and flexibility in operation. A timer, which is one shot operation, opens the shutter thus allowing the beans to fill the cavity in the grinder head below the shutter. The shutter then is closed. Grinding operation would then be initiated to grind the entire volume of beans which occupy the cavity. This volume is the minimum reliable volume since any volume less than the volume of the cavity can only be portioned by either reducing the time period for shutter operation or setting a shorter grind period which would not grind all the coffee beans given the grinder head. The former would create high repeatability errors and the latter would result in contamination of the coffee since the grinder serves two hoppers each of which would likely contain different types of coffee beans. In practice, however, the sliding valve arrangements as described above are generally used where the minimum reliable volume is exceeded by increasing the time the slide is left open.
The total volume of the coffee ground can be expressed in approximate terms by the following simple relationship: EQU y=kt+b
where b is the minimum volume, k is the volumetric rate at which the grinder operates, and t is the time required for grinding the total volume. In the situation in which the total volume is equal to the minimum volume, e.g., the volume occupied by about one and one half ounces of coffee beans, the shutter might be open for 2 seconds and the grinder could be operated for slightly less than 4 seconds (assuming a grind rate of 0.4 ounces per second). However, in practice the grinder would be operated for a much longer time period, e.g., ten seconds, to ensure absolutely that all of the beans were ground. This extended time period is over six seconds longer than required, but repeatability is accurate and can be shown to be about 3 to 4%.
When it is desired to have a larger volume to be round, for example, a throw of about three ounces, the shutter time can be extended and determined via the expression above to be about 4 seconds. In this situation the grinder is turned on simultaneously with the opening of the shutter and continues to grind for ten seconds after the shutter has been closed. The longer the shutter is left open, the greater the coffee portion is. As the "kx" portion of the equation becomes dominant, i.e. for kx&gt;&gt;b, the operation of the grinder apparatus becomes closer to a pure time controlled operation.
A problem of consistency occurs when the desired vol becomes less than the cavity of the grinder. When the shutter is operated rapidly providing too short an opening time for the beans to fill the grinder cavity, significant errors are possible. The varying size, surface structure and shape of the coffee beans all contribute to the flowability of the bean stream emanating from the hopper into the grinder. Dispensing errors up to 30 to 50% may result when such smaller volumes are required. Moreover, the addition of hardware to control the shutter speed at higher speed ranges increases the expense of the grinding system and the complexity thereof.
The above problem has become exacerbated as the consuming public has changed its taste preferences to finer grounds and darker roasted and flavored coffees. The weights of coffee used to brew a typical 12 cup decanter have been steadily declining since 1975 when the recommended portioning was from 2.75 to 3.0 ounces to 1990 where the proper portioning can be a throw as small as 0.75 ounces. Moreover, darker roasted coffees tend to have beans which are larger, increased surface oils, and are more friable. Flavored coffees are usually coffees of which the beans have been coated with flavorants such as chocolate, vanilla, sugar, cream and the like which lead to alterations in the flow patterns of the coffee beans when dispensed. These preference changes by the consuming public have made it much more difficult for existing bean dispensing apparatus to provide the required portioning consistency between grinding operations. For example, shutter mechanisms used to dispense a throw of beans of 1.5 ounces or less give rise to dramatic and undesirable increases in errors in repeatability, often at unacceptable levels approaching 30%.
Clearly, a more precise means of metering the coffee beans is needed such as provided by auger or screw conveyor mechanisms. One interesting patent is U.S. Pat. No. 3,228,561 issued Jan. 11, 1966 to Rosenburg illustrates two augers or screw conveyors being used with a dual hopper apparatus in which the material in the hoppers is selectively moved by the augers to a grinding mechanism. The augers provide a mechanism for more precise metering of the materials to the grinding operation and subsequent mixing. This use of augers is typical and employed to a large extent in the chemical industry where careful attention is being paid to the amounts of materials being transported and mixed.
Auger mechanisms for moving coffee grounds have been known for some time. One example is disclosed in U.S. Pat. No. 4,688,474 issued Aug. 25, 1987 to Siegfried Anderl in which a plurality of ground coffee containers communicate through respective augers to a coffee percolator. Still another U.S. Pat. No. 4,493,249 issued Jan. 15, 1985 to Stover discloses the movement of flaked or freeze dried coffee material to a bowl containing hot water for mixing and dissolving. Thus, it is clear that augers have found acceptance for use with coffee brewing mechanisms. In contrast, however, use of augers with coffee beans has been limited by the industry due to the problems experienced with handling of the beans. The size of coffee beans are orders of magnitude larger than ground coffee particles which give rise to significant metering problems when egress of beans into the core flow is disturbed. Additionally, the oils inherently present in coffee beans introduce problems not faced by existing auger conveyors used for moving coffee grounds.
Thus, it would be highly desirable to have a coffee bean movement mechanism which could provide the superior metering capability of an auger conveying mechanism without the serious handling drawbacks presently experienced.